यो गुरुः स शिवः प्रोक्तो यः शिवः स गुरुः स्मृतः | 42
One who is Guru is also called Shiva (a disciple should see the Guru as God); one who is Shiva is also considered to be the Guru (God and Guru should be the same element for a disciple).

The same sentence, the same half-shloka also appears in Guru Gita (18th of 214 shlokas), which is a part of Uttarakhand of Skanda Purana. There too, God says this same thing to the supreme Goddess. For this very reason, Guru Gita is also called Shiva Gita.

This sentence also repeats in Chapter 12 of Kailasa Samhita, where Kartikeya tells this to sage Vamadeva. It also appears in Vayayiya Samhita, later part, Chapter 15, shloka 20, where Upamanyu says this to Lord Krishna.